1. Field of the Invention
The present invention relates to a friction drive device that is equipped a drive roller and a driven roller that are pressed to contact with each other so as to transfer mechanical power between them at a reduction ratio or an overdrive speed ratio.
2. Description of the Related Art
A friction drive device of this kind is disclosed in Japanese patent laying-open publication No. 2001-173743. In this drive device, a drive roller and a driven roller are pressed to contact with each other so that friction force caused at their contact point enables the rollers to transmit mechanical power between them. The press force are obtained by adjusting bolts of a roller support portion so that their center distance becomes smaller.
Another friction drive device of this kind is disclosed in Japanese patent laying-open publication No. 2002-349654. This drive device has a wedge roller arranged between drive and driven rollers to have a contact point of the drive roller and wedge roller and another contact point of the driven roller and the wedge roller in a manner that their tangents at the contact points differ from each other.
The above known conventional friction drive devices, however, encounter a problem that lifetime of the roller and bearings of the rollers are shortened and power transmission efficiency between the rollers deteriorates. This reason comes from the following: in the former drive device, the press force between the rollers does not change according to the transmission torque between the rollers, and can not be adjusted optimally both in low and high torque transmitting ranges. In the latter drive device, the drive and driven rollers are depressed to cause elastic deformation at their contact points by further ingression of the wedge roller into the drive and driven rollers when the press force becomes larger according to the transmitting torque between the rollers.
In order to avoid the above problem that the press force can not be optimally controlled according to the transmission torque between the rollers, Japanese patent laying-open publication No. (Hei) 8-277896 discloses a friction drive device having an air cylinder to control press force at a minimum optical level according to transmitting torque between a drive and driven rollers. This drive device, however, needs the air cylinder, a pump to supply pressurized air to the cylinder, and a control unit to control the cylinder, which results in deterioration of total efficiency and high manufacturing costs.
It is, therefore, an object of the present invention to provide a friction drive device which overcomes the foregoing drawbacks and can obtain press force varied according to transmitting torque without an expensive or complicate device and thereby improve its lifetime and its power transmission efficiency.